/diy/ - Do It Yourself

disclaimer: if /ohm/ is anything like /dpt/, you get a bunch of posts from drooling retards asking the most inane shit every day. this is one of those posts.

hi /ohm/. I recently got interested in electronics after finding some old components (assorted ICs, jumper cables, breadboards, resistors, etc etc). right now I'm reading this https://www.allaboutcircuits.com/textbook/, which is probability shit if electronics content is like programming content (if it's not available as a book, it's worthless), but I'm liking it so far and it doesn't seem to be saying anything dumb (from what I can tell with my previous physics/digital circuits/google knowledge). hopefully I didn't pick too terrible of a resource to read.

I'm stuck trying to reason about the voltage between points in a few circuits. pic related.
in circuit A, the voltage between any points in the set {1, 3} and the points in {2, 4} would be 9v (ignoring the sign), correct? the current is 0, because the circit is open.
in circuit B, I reason that the voltage between points 1 and 2 is 0v, because they are electrically common, and through this reasoning, 1 and 2 are electrically common with 3 and 4 (respectively) and so the voltage between 3 and 4 must be 0v. but on the other hand, it would seem like the voltage between 3 and 4 should be 9v, because that's the electrical potential difference between the two terminals of the source. I'm very much leaning towards it being 0v (ignoring the resistance of the wires), with the current being +inf. but why is my second line of reasoning flawed?
circuit C is what lead me to think about all this, because I was reading about parallel circuits and wondered what would happen if I simply removed one of the resistances. I'm assuming that the answer to my question about circuit B will help me understand circuit C, but I included it anyway (in case anyone has anything to comment on it).

(cont)